docs: def form_number(self):
  		"""
  		Returns the model's form number, a helpful heuristic for classifying tides.
  		"""
  		k1, o1, m2, s2 = (
  			np.extract(self.model['constituent'] == c, self.model['amplitude'])
  			for c in [constituent._K1, constituent._O1, constituent._M2, constituent._S2]
  		)
  		return (k1+o1)/(m2+s2)